/*
 * Portions Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * This software may be used and distributed according to the terms of the
 * GNU General Public License version 2.
 */

/*
 mpatch.c - efficient binary patching for Mercurial

 This implements a patch algorithm that's O(m + nlog n) where m is the
 size of the output and n is the number of patches.

 Given a list of binary patches, it unpacks each into a hunk list,
 then combines the hunk lists with a treewise recursion to form a
 single hunk list. This hunk list is then applied to the original
 text.

 The text (or binary) fragments are copied directly from their source
 Python objects into a preallocated output string to avoid the
 allocation of intermediate Python objects. Working memory is about 2x
 the total number of hunks.

 Copyright 2005, 2006 Olivia Mackall <olivia@selenic.com>

 This software may be used and distributed according to the terms
 of the GNU General Public License, incorporated herein by reference.
*/

#include <stdlib.h>
#include <string.h>

#include "eden/scm/sapling/bitmanipulation.h"
#include "eden/scm/sapling/mpatch.h"

static struct mpatch_flist* lalloc(ssize_t size) {
  struct mpatch_flist* a = NULL;

  if (size < 1) {
    size = 1;
  }

  a = (struct mpatch_flist*)malloc(sizeof(struct mpatch_flist));
  if (a) {
    a->base = (struct mpatch_frag*)malloc(sizeof(struct mpatch_frag) * size);
    if (a->base) {
      a->head = a->tail = a->base;
      return a;
    }
    free(a);
  }
  return NULL;
}

void mpatch_lfree(struct mpatch_flist* a) {
  if (a) {
    free(a->base);
    free(a);
  }
}

static ssize_t lsize(struct mpatch_flist* a) {
  return a->tail - a->head;
}

/* move hunks in source that are less cut to dest, compensating
   for changes in offset. the last hunk may be split if necessary.
*/
static int gather(
    struct mpatch_flist* dest,
    struct mpatch_flist* src,
    int cut,
    int offset) {
  struct mpatch_frag *d = dest->tail, *s = src->head;
  int postend, c, l;

  while (s != src->tail) {
    if (s->start + offset >= cut) {
      break; /* we've gone far enough */
    }

    postend = offset + s->start + s->len;
    if (postend <= cut) {
      /* save this hunk */
      offset += s->start + s->len - s->end;
      *d++ = *s++;
    } else {
      /* break up this hunk */
      c = cut - offset;
      if (s->end < c) {
        c = s->end;
      }
      l = cut - offset - s->start;
      if (s->len < l) {
        l = s->len;
      }

      offset += s->start + l - c;

      d->start = s->start;
      d->end = c;
      d->len = l;
      d->data = s->data;
      d++;
      s->start = c;
      s->len = s->len - l;
      s->data = s->data + l;

      break;
    }
  }

  dest->tail = d;
  src->head = s;
  return offset;
}

/* like gather, but with no output list */
static int discard(struct mpatch_flist* src, int cut, int offset) {
  struct mpatch_frag* s = src->head;
  int postend, c, l;

  while (s != src->tail) {
    if (s->start + offset >= cut) {
      break;
    }

    postend = offset + s->start + s->len;
    if (postend <= cut) {
      offset += s->start + s->len - s->end;
      s++;
    } else {
      c = cut - offset;
      if (s->end < c) {
        c = s->end;
      }
      l = cut - offset - s->start;
      if (s->len < l) {
        l = s->len;
      }

      offset += s->start + l - c;
      s->start = c;
      s->len = s->len - l;
      s->data = s->data + l;

      break;
    }
  }

  src->head = s;
  return offset;
}

/* combine hunk lists a and b, while adjusting b for offset changes in a/
   this deletes a and b and returns the resultant list. */
static struct mpatch_flist* combine(
    struct mpatch_flist* a,
    struct mpatch_flist* b) {
  struct mpatch_flist* c = NULL;
  struct mpatch_frag *bh, *ct;
  int offset = 0, post;

  if (a && b) {
    c = lalloc((lsize(a) + lsize(b)) * 2);
  }

  if (c) {
    for (bh = b->head; bh != b->tail; bh++) {
      /* save old hunks */
      offset = gather(c, a, bh->start, offset);

      /* discard replaced hunks */
      post = discard(a, bh->end, offset);

      /* insert new hunk */
      ct = c->tail;
      ct->start = bh->start - offset;
      ct->end = bh->end - post;
      ct->len = bh->len;
      ct->data = bh->data;
      c->tail++;
      offset = post;
    }

    /* hold on to tail from a */
    memcpy(c->tail, a->head, sizeof(struct mpatch_frag) * lsize(a));
    c->tail += lsize(a);
  }

  mpatch_lfree(a);
  mpatch_lfree(b);
  return c;
}

/* decode a binary patch into a hunk list */
int mpatch_decode(const char* bin, ssize_t len, struct mpatch_flist** res) {
  struct mpatch_flist* l;
  struct mpatch_frag* lt;
  int pos = 0;

  /* assume worst case size, we won't have many of these lists */
  l = lalloc(len / 12 + 1);
  if (!l) {
    return MPATCH_ERR_NO_MEM;
  }

  lt = l->tail;

  /* `len - 11` because we access the pos + 11th byte */
  while (pos >= 0 && pos < len - 11) {
    lt->start = getbe32(bin + pos);
    lt->end = getbe32(bin + pos + 4);
    lt->len = getbe32(bin + pos + 8);
    lt->data = bin + pos + 12;
    pos += 12 + lt->len;
    if (lt->start > lt->end || lt->len < 0) {
      break; /* sanity check */
    }
    lt++;
  }

  if (pos != len) {
    mpatch_lfree(l);
    return MPATCH_ERR_CANNOT_BE_DECODED;
  }

  l->tail = lt;
  *res = l;
  return 0;
}

/* calculate the size of resultant text */
ssize_t mpatch_calcsize(ssize_t len, struct mpatch_flist* l) {
  ssize_t outlen = 0, last = 0;
  struct mpatch_frag* f = l->head;

  while (f != l->tail) {
    if (f->start < last || f->end > len) {
      return MPATCH_ERR_INVALID_PATCH;
    }
    outlen += f->start - last;
    last = f->end;
    outlen += f->len;
    f++;
  }

  outlen += len - last;
  return outlen;
}

int mpatch_apply(
    char* buf,
    const char* orig,
    ssize_t len,
    struct mpatch_flist* l) {
  struct mpatch_frag* f = l->head;
  int last = 0;
  char* p = buf;

  while (f != l->tail) {
    if (f->start < last || f->end > len) {
      return MPATCH_ERR_INVALID_PATCH;
    }
    memcpy(p, orig + last, f->start - last);
    p += f->start - last;
    memcpy(p, f->data, f->len);
    last = f->end;
    p += f->len;
    f++;
  }
  memcpy(p, orig + last, len - last);
  return 0;
}

/* recursively generate a patch of all bins between start and end */
struct mpatch_flist* mpatch_fold(
    void* bins,
    struct mpatch_flist* (*get_next_item)(void*, ssize_t),
    ssize_t start,
    ssize_t end) {
  ssize_t len;

  if (start + 1 == end) {
    /* trivial case, output a decoded list */
    return get_next_item(bins, start);
  }

  /* divide and conquer, memory management is elsewhere */
  len = (end - start) / 2;
  return combine(
      mpatch_fold(bins, get_next_item, start, start + len),
      mpatch_fold(bins, get_next_item, start + len, end));
}
